Sonic Heroes was repetitive (you had to play through what's essentially the exact same set of levels four different times to unlock the true ending), and Sonic Unleashed felt like two separate games stuck together. The daytime stages were seen as solid, but the nighttime stages slowed the game down severely and turned it into a dollar store E-rated version of God of War.
At least, that's what I recall of what the common issues were; it's been a while since I've played them, so I could be way off. I do recall Sonic Heroes' controls feeling really slippery, though maybe that was just me.